Polyurethane prepolymers have many different industrial applications. Numerous systems are formulated from isocyanate systems containing volatile isocyanate monomers such as methylene diphenyl diisocyanate (MDI). Such use includes foams, adhesives, undercarriage coatings, vehicular silencer systems, elastomer elements and articles. The subject of interest by industrial circles lies in minimising the risk of exposure to volatile isocyanate monomers, which may be harmful to the health of staff. Thus, there exists a need to reduce the concentration of isocyanate monomers in precursors and polyurethane prepolymers.
One solution known from the current state of the art is removal of volatile isocyanate monomers from the prepolymer system using a distillation technique such as a wiped film evaporator. See Anderson et al, U.S. Pat. No. 5,441,808, column 6, lines 29-34. This solution requires an additional unit operation, increasing investment and operational costs for formulating such prepolymers.
Another solution known to the current state of the art encompasses formulating prepolymers in a two step process, where in the first step polyhydric alcohols react with asymmetric isocyanate with isocyanate groups of different reactivity. Polyols preferentially react with the more reactive group. This process continues until the theoretical quantity of the faster-reacting isocyanates reacts with polyhydric alcohols. Then the product of the reaction is subject to a reaction with an even faster-reacting symmetric isocyanate in order to form the prepolymer. The two phase process also requires an additional unit operation. The extra step and additional cost required to handle the second isocyanate significantly increases the costs of this process. See Bolte et al, U.S. Pat. No. 6,515,164 and Bauriedel, U.S. Pat. No. 4,623,709. This process requires very precise processing conditions to be determined and due to the chain growth limitation provides lower flexibility with reference to the target prepolymer viscosity and prevents specialists from readjusting the prepolymer to satisfy the defined target properties.
European Patent Application No. EP2481764 discloses the use of a monofunctional alcohol in a prepolymer composition, with a molecular mass of at least 130 g/mol and a paraffin structure, favourable for 2-ethylhexyl alcohol. In the aforementioned patent application, two components are formed in the first step. Component A was formulated by mixing at room temperature for 20 min, adding the following ingredients: castor oil, ethylene glycol, dimorpholinodiethylether (DMDEE), TCPP and silicone-surfactant (Tegostab B8870). Component B was formulated separately by mixing at room temperature for 35 minutes: crude MDI (Supersec 5025) with 2-ethylhexyl alcohol, and once the two components are mixed, an NCO terminated TDI prepolymer was added and then mixed. Then Components A and B were put into a valved can and porophors were injected. The next phase involved shaking the can for 2-3 minutes. The aforementioned method is a two step method for formulating OCF foam with low MDI content. However, it is relatively complicated. Due to the low MDI content, being NCO prepolymer solvent, the foam viscosity is increased.
International Patent Application No. PCT/PL2013/050002 talks about a one component isocyanate prepolymer mixture for formulating a polyurethane product in a single step process, with monomeric methylene diphenyl diisocyanate (MDI) content in the said mixture not exceeding 3% of the total weight containing tertiary monofunctional alcohol, an isocyanate component or isocyanate component mixture, propellant, catalyst, stabiliser and one or more polyols. The said prepolymer mixture uses tertiary alcohols, which prior to their use in the process require melting prior to being used in the primary reactor. Thus, polyurethane prepolymers are required which contain low concentrations of volatile isocyanate monomers, whereas the prepolymer may be formulated using standard, commercially available isocyanate monomer systems with the application of conventional techniques. Compositions and foam system containing such prepolymer are also required. Furthermore, such prepolymers are required, which may be formulated without the need to use additional separate unit operations or complex process steps which have to be separated due to their exothermic properties, which may cause the occurrence of excessive, unwanted pressure in the can, even during the step of preparing substrates for the prepolymer formulation reaction, i.e., melting solid tertiary alcohols. Additionally, it is required to eliminate high cost compounds such as the TDI prepolymer. Furthermore, the obtained prepolymer and foam should exhibit the standard yield of the end foam. Unexpectedly, the present invention solves the above problems.